In addition to conventional operating procedures, it is also possible to operate gasoline engines in a so-called lean operation. During the lean operation, the combustion takes place in the cylinders of the gasoline engine in the presence of excess air, which is typical for diesel engines. To maintain the ignitability, the fuel is injected into the cylinders in such a way that at the ignition timing of the spark plug, an ignitable air/fuel mixture is present in the area of the spark plug. Due to the great quantity of excess air and the high temperatures arising during the combustion, the nitrogen present in the air reacts with the atmospheric oxygen to form nitrogen oxides.
Nitrogen oxides in the exhaust gas are classified as harmful. Nitrogen oxide catalytic converters are provided to reduce nitrogen oxides with the aid of chemical reduction. From time to time, the nitrogen oxide catalytic converters must be regenerated to reduce the accumulated nitrogen oxides. In particular, in the case of a great nitrogen oxide discharge by the internal combustion engine, more nitrogen oxide is stored in the catalytic converter per time unit. Thus, the intervals between the regenerations become shorter.
The regenerations of the catalytic converters require a combustion under excess fuel (lambda smaller than 1). The combustion under excess fuel results in a greater portion of carbon monoxide, hydrogen, and/or hydrocarbons in the combustion exhaust gas which are used as reduction agents for the regeneration. Combustion under excess fuel results in a significant fuel inefficiency. To reduce the quantity of the nitrogen oxides discharged by the internal combustion engine, an exhaust gas recirculation is usually provided which admixes combustion exhaust gas to the ambient air supplied to the cylinders. The combustion exhaust gas is used in the combustion chamber of the cylinders as an inert gas, i.e., chemically, it behaves generally neutrally.
The metering of the quantity of the recirculated exhaust gas usually takes place in a pilot-controlled manner. The quantity of the recirculated exhaust gas actually supplied to the combustion chamber is calculated based on balancing the air and exhaust gas quantities with the aid of the air mass flow meter and the pressure sensor provided in the air supply system. During a completely or almost completely dethrottled engine operation, such as the one occurring during lean operation, the model on which the calculation is based, however, results in excessively high tolerances so that the calculated exhaust gas recirculation rate may considerably deviate from the actual exhaust gas recirculation rate. This effect is intensified even more when the internal combustion engine is provided with a charging device, e.g., an exhaust gas turbocharger, so that the inaccuracies to be expected in the calculation of the exhaust gas recirculation rate noticeably delimit the usable and fuel-efficient range for the operation of the internal combustion engine during lean operation.
The delimitation of the usable range for the settable exhaust gas recirculation rate results from the combustion properties deteriorating considerably to the point of misfires already in the case of an exhaust gas recirculation rate which is slightly higher than its optimum. If the actually supplied exhaust gas recirculation rates are too low, the nitrogen oxide emissions increase to a disproportionately high extent, thus resulting in frequent regenerations of the nitrogen oxide catalytic converter and a significant fuel inefficiency.
It is an object of the present invention to provide an improved method and an improved device for setting the exhaust gas recirculation rate to a usable and fuel-efficient range.